Ceramic chip capacitors are widely used in hybrid and printed circuit modules because they offer ruggedness, volumetric efficiency, wide available range and attractive cost. Generally, the chips are fabricated by interleaving rectangular electrode plates and dielectric layers, alternately attaching the plates to two termination bands on the opposing ends of the chip. A substantial percentage of ceramic chip capacitors are fabricated using barium titanate ceramic as the dielectric material with a typical thickness of 1 mil. Palladium silver is frequently used for the plates and thicknesses in the order of 0.1 mil are typical. It is common practice to use solder joints to connect the termination bands to the circuit of the module.
The primary means of soldering chip capacitors and chip components in general, is with a hand soldering iron which can damage not only the component but also the module. For example, if heat is permitted to conduct into the component, internal pressure may develop from trapped gas molecules within the component that are not able to escape as rapidly as they expand. Also, if the adjoining layers of the component have a mismatch in coefficients of thermal expansion, shear stresses develop within the component. Further, even if the component materials are selected to have similar coefficients of thermal expansion, they likely will exhibit different thermal conductivities such that a temperature gradient will exist between the layers and cause shear stresses of expansion,. Also, stress can be created in the solder joints substantially caused by two conditions. First, during the heating process, the chip is free to expand but the module area is hindered from expansion by the mechanical restraint provided by the cooler surrounding material of the module; compressive stress occurs in the local area of the module but no expansion. Then, upon cooling to room temperature, the chip and solder joints are in a state of tensile stress as the chip is partially restrained from decreasing to its original length by the solidified solder. Second, if the joints are soldered one at a time, substantial stresses occur in the joints and the chip caused by even minimal contact with the chip during the formation of the second solder joint.
The internal component stresses heretofore described can cause component failures and thereby substantially reduce the reliability of a module. Examples of structural defects caused by stress are delaminations, material crumpling, voids, and cracks. The effects may, for example, be short and open circuits or changes in capacitive properties. Also, stress on the soldering joints may result in a poor connection. The prior art includes various techniques for soldering chip capacitors, the most common of which is the use of hand soldering iron. Other methods utilize flame, hot air, or focused radiant energy. However, a need still exists for a means and method for soldering chip capacitors to modules so as to minimize the internal component stresses and the stress on the solder joints.